Description of the Prior Art
The German Pat. No. 1,490,713 discloses a cold conductor with contact means which are free of blocking layers wherein the contact means comprise two layers of different materials. On the surface of the cold conductor at the points to be contacted there is arranged an indium or indium-gallium layer and upon the latter is arranged a layer of stoving silver preparation which is stoved in an oxidizing atmosphere. The difficulties involved in the production of this known cold conductor is that the two process steps for the production of the contact platings must be very carefully adapted to one another if it is to be certain that the values for the bonding strength and the freedom of blockages are to be reproducible. In addition, the process for the production of the cold conductor is difficult to automate and the contact platings consist of expensive materials.
U.S. Pat. No. 3,586,534 also discloses contact means of cold conductors which are free of blocking layers. In this case, the surfaces which are to be metallized are sensitized and activated, and then a currentless deposition of a nickel-phosphorus alloy is effected. The sensitization is effected by submerging the cold conductors in a tin (II)-chloride solution and then activation is effected by submerging in a palladium (II) - chloride solution. Currentless deposition of a nickel-phosphorus alloy is made upon the sensitized and activated surface in a bath which contains nickel (II) - chloride, sodium hypophosphite as reduction agent and sodium citrate as stabilization agent. The difficulties involved in the production of these contacts is that prior to submerging the cold conductor bodies in the sensitization bath, it is necessary to cover those surface parts where no contact plating is to be provided. Also, by this method it is difficult to dose the palladium chloride which adheres to the surface and which determines the bonding strength of the nickel-phosphorus layer. In addition, the danger exists that the palladium chloride will enter and decompose the nickel bath.